A stereo imaging system includes a stereoscopic camera which has two image capturing elements for capturing left and right stereo images. Details for such a stereo imaging system may be found, for example, in U.S. Pat. No. 6,720,988 entitled “Stereo Imaging System and Method for Use in Telerobotic System.”
FIG. 1 illustrates a schematic of the stereo geometry for two image capturing elements, e.g., left and right optical lens 101, 102, which are separated by a baseline distance “b”. Left and right image planes 121, 122 are shown at a focal length “f” (i.e., a depth at which the left and right images are focused). The image planes 121, 122 represent stereo images that are captured by the lens 101, 102 and are bounded by their fields of view. The focal length may be adjusted within a focusing range, but the baseline distance is fixed for the stereoscopic camera.
A point “P” at a depth “Z” from the lens 101, 102 is seen at different points on the image planes 121, 122. In particular, the point “P” is projected at a position “d1” on the left image plane 121 and projected at a position “d2” on the right image plane 122. The difference or disparity “D” between the two positions “d2” and “d1” can be determined from the following well-known relationship:
                              D          b                =                  f          z                                    (        1        )            
Thus, as the depth “Z” gets smaller and smaller, the disparity “D” gets larger and larger.
Stereo images captured by the stereoscopic camera are displayed on a stereo viewer. As an example, the stereo viewer may have left and right display screens upon which left and right stereo images are respectively displayed. The stereo viewer in this case, may also have left and right eyepieces through which a user places his/her left and right eyes to respectively view the left and right display screens.
When objects are viewed at very close range by the stereoscopic camera, even after or while adjusting the camera focus, the user may have difficulty fusing his/her eyes on an image of the object being displayed on the stereo viewer due to the large disparity “D” between corresponding points on the left and right display screens. In addition, after the camera focus control has reached the end of its range, a practical limit may be placed on how close an object may be viewed relative to the stereoscopic camera.
To address this problem, another stereoscopic camera with a smaller baseline “b” (i.e., closer spacing between the image capturing elements) may be used at very close range to reduce the disparity “D” between displayed stereo images and consequently, allow a user to comfortably see the stereo images being displayed on the stereo viewer. However, the use of multiple stereoscopic cameras with different baselines adversely adds to the cost of the stereo imaging system and increases the difficulty of its use by an operator.